Because...
Because, the source of legislation and ahkam, in the light of definite shar'i dicta, is God alone, and no other being. No hukm or law except that which is legislated by Him can be given the status of a shar'i hukm. Even the Prophet (S) cannot be considered as a source of legislation of the ahkamof the Shari'ah.
The belief cherished by the majority of scholars of the Sunni community that the Prophet (S) himself sometimes exercised ijtihadand himself legislated laws according to his own ray and subjective opinion in some issues and problems, and the traditions narrated in this regard, have no validity whatsoever (an elaborate refutation of this view will be given in the article "The Prophet (S) and Ijtihad").
Accordingly, when the Prophet (S) cannot be considered as the source of the tashri` of ahkam, is it possible that subjective views and opinions of human individuals with no links with wahy, and whose character, behaviour and speech are not considered a norm and model for others, be considered laws of God and they themselves as legislators of the ahkam of the Shari'ah? No doubt, it is possible that occasionally ijtihadmight have figuratively been referred to as tashri` and legislation.
For instance, the renowned scholar Abu Ishaq Ibrahim al-Shatibi al-Gharnati al-Maliki (d.790/1388), the author of al-Muwafiqat, has also named the task of a mujtahid as tashri` and legislation. No doubt, his usage carries only a figurative sense; for naming the activity of a mujtahid as legislation was for the reason that ijtihad(i.e.
application of the usul of the ahkam and the general principles for deriving other ahkam regarding emergent issues and new problems) is an effort to discover a shar`i hukm, thereby discovering the intent of the Lawgiver and obtaining the hukm of God. Then, in reality, it amounts to calling `legislator', in a figurative sense, one who discovers a law. Since in Islamic fiqh there is in fact no provision for anybody except God to lay down laws.
Therefore, the Shari'ah is made up of the injunctions and commandments that were revealed to the Prophet (S) by God Almighty through the agency of Jibrail. There are verses in the Quran which confirm this fact; they will be discussed in the article entitled "The Prophet (S) and Ijtihad".